To unambiguously distinguish the different scenarios discussed above, it is vital to resolve the AFM structure of the HSL, especially the moment orientation. However, directly probing the AFM structure and transition is challenging as the limited volume of ultrathin film samples precludes many methods applicable to the bulk, such as neutron scattering [1]}, [2]}. RXMS offers an ideal solution thanks to its resonant enhancement and high photon energy in the hard x-ray regime. Figure 3(a) shows the spectrum of the (-0.5 0.5 17.5) peak observed under the vertical scattering geometry in the \(\sigma - \pi ^{\prime }\)  channel, which shows a typical magnetic line shape at the Ir \(L_3\) -edge [3]}, [4]}, [5]}, [6]}, [7]}. Magnetic peaks at half order of \(H\)  and \(K\)  are characteristic of a nearest-neighbor AFM order within the \(ab\) -plane, consistent with the expected magnetic ground state of a pseudospin-half square lattice. Figure 3(b) shows the temperature dependence of the intensity, which reveals the transition temperature \(\it {T}\)\(_N\)  = 120 K. This is also the only observed transition, consistent with the magnetization result. Interestingly, including (-0.5 0.5 17.5), magnetic peaks are only found at the half-order positions of \(L\)  as shown in Fig.3(c), suggesting that the magnetic cell consists of two hybrid supercells that are magnetically out-of-phase from each other along the \(c\) -axis. To probe the AFM moment direction, we measured the (1.5 1.5 1.5) magnetic peak under the horizontal geometry with polarization analysis. Figure 3(d) and (e) show that the peak only has intensity in the \(\pi - \pi ^{\prime }\)  channel but no intensity in the \(\pi - \sigma ^{\prime }\)  channel, suggesting that the AFM moments of the entire HSL are predominantly in-plane and the \(c\) -axis component is weak (if not zero) [8]}, [9]}.
